PHOTO 3. The ETRX2 is now perfboard ready. I soldered
the SMT pads to the adjacent 64-pin IC lugs. Both the ETRX2

and the 64-pin socket are upside down leaving the 64-pin
socket holes available for the SIP header pins.

PHOTO 2. The ETRX2 can be electrically attached to the host
and support circuitry using SMT pads or the 1. 27 mm pin area.
The recommended pin arrangement is specified in the ETRX2
datasheet as Harwin part number M50-3601042.

command set to put your application online. Our goal is to
put a self-healing, sensor-friendly network together using a
PIC microcontroller and a minimum of firmware. Let’s put the
four-wheeler in the bed of the truck and get on the highway.

Putting a Network Node Together

The ETRX2’s heart is composed of the EM250 and its
supporting RF circuitry. Unless you have an interest in EM250
primitives, there is no need for us to expound upon the design
of the EM250 IC or its supporting circuitry. All we really need
is some direction as to how to access the ETRX2’s built-in networking capabilities. It would also be a good thing if we were
given some direction as to how to attach the ETRX2 to our PIC.

Photo 2 is a solder-side view of the ETRX2. The smaller
1. 27 mm-pitch pad area connects electrically to the SMT pads
around the perimeter of the ETRX2. The inclusion of those pads
allows the ETRX2 to be “plugged” into a circuit. Obviously,
the SMT pads are used for applications that require a permanent
installation. All of the ETRX2’s perimeter SMT pads are
pitched at 0.1 inch ( 2. 54 mm). Unfortunately,
the width between pins 14 and 21 — which are
along the short edge — is not pitched in an exact
multiple of 0.1 inches. Since the circuitry must
be simple, the plan was to not fabricate a PCB
for our ETRX2-based node. Instead, I was
planning on putting the PIC and the ETRX2
module down on a standard 0.1 inch perf
board. Now that we know that we can’t
simply socket the ETRX2 for inclusion in our
application, we’ll have to go with Plan 9 from
Outer Space. (For you sci-fi fans, Plan 9 is
considered the worst sci-fi movie of all time.
Aliens that can revive the dead arrive on Earth
in an attempt to keep Earth’s scientists from
discovering that they could use sunlight to
create a universe-destroying bomb called solar-

bonite.) Our Plan 9 doesn’t involve raising the dead or
saving the universe. Using portions of a Jameco 64-pin IC
socket (Jameco part number 200707) and a straight SIP
header (Digi-Key part number ED7264-ND), I managed to
“socket” the ETRX2 as you can see in Photo 3. After sawing
the 64-pin IC socket to size, I turned the ETRX2 over on its
back and centered it between the 0.9 inch pitch rows of
the 64-pin IC socket’s pins. The IC socket is also upside
down in Photo 3. With the ETRX2’s SMT pads aligned with
the pins of the socket, I proceeded to solder short wires
between each ETRX2 SMT pad and its adjacent IC socket
pin. The SIP header is designed to fit one side of its pins
into a standard IC socket, and the other side of pins into a
larger header-sized mounting hole. That works out fine as
the perfboard holes are designed to accept standard 0.025
inch header posts. The socket is completed by installing the
cut-to-length SIP header pieces into holes of the 64-pin IC
socket. Note that all of the SMT pins along the edges of the
ETRX2 are not connected to an IC socket pin of our makeshift
ETRX2 socket. That’s because we only need to access ETRX2
pins 4 through 14, and 21 through 31. We also need to
adhere to the ETRX2 datasheet’s antenna rule which tells us
to keep the area under the ceramic antenna clear of traces

FIGURE 1. The unused pins are along the short
edges of the ETRX2. Pins 1, 2, 3, and 32